System and method for transfering volume ownership in net-worked storage

ABSTRACT

A system and method for transferring ownership of a volume in a networked storage environment. The method first moves the disks that comprise the volume being transferred from a source-owned state to an un-owned state. Next the disks are moved to a destination-owned state. Each step of the transfer process completes on all disks before the next step begins to ensure consistency and disks that have conflicting owners. Alternate embodiments include logging by the source and destination nodes to permit a resumption of the transfer if one node becomes inactive during the transfer.

RELATED APPLICATIONS

This application is related to the following United States PatentApplications:

Ser. No. 10/027,457 entitled SYSTEM AND METHOD OF IMPLEMENTING DISKOWNERSHIP IN NETWORKED STORAGE, by Susan M. Coatney et al.

Ser. No. 10/027,330 entitled SYSTEM AND METHOD FOR STORING STORAGEOPERATING SYSTEM DATA IN SWITCH PORTS, by Susan M. Coatney et al.

Ser. No. 10/027,330 entitled SYSTEM AND METHOD FOR ALLOCATING SPAREDISKS IN NETWORKED STORAGE, by Alan L. Rowe et al.

FIELD OF THE INVENTION

The present invention relates to networked file servers, and moreparticularly to transferring volume ownership in networked file servers.

BACKGROUND OF THE INVENTION

A file server is a special purpose computer that provides file servicerelating to the organization of information on storage devices, such asdisks. The file server or filer includes a storage operating system thatimplements a file system to logically organize the information as ahierarchical structure of directories and files on the disks. Each “ondisk” file may be implemented as a set of data structures, e.g., diskblocks, configured to store information. A directory, on the other hand,may be implemented as a specially formatted file in which informationabout other files and directories are stored.

A filer may be further configured to operate according to aclient/server model of information delivery to thereby allow manyclients to access files stored on a server. In this model, the clientmay comprise an application, such as a database application, executingon a computer that connects to the filer over a computer network. Thiscomputer network could be a point to point link, a shared local areanetwork (LAN), a wide area network (WAN) or a virtual private network(VPN) implemented over a public network such as the Internet. Eachclient may request the services of the file system on the filer byissuing file system protocol messages (typically in the form of packets)to the filer over the network.

The disk storage typically implemented has one or more storage “volumes”comprised of a cluster of physical storage disks, defining an overalllogical arrangement of storage space. Currently available filerimplementations can serve a large number of discrete volumes (150 ormore, for example). Each volume is generally associated with its ownfile system. The disks within a volume/file system are typicallyorganized as one or more groups of Redundant Array of Independent (orInexpensive) Disks (RAID). RAID implementations enhance the reliabilityand integrity of data storage through the redundant writing of datastripes across a given number of physical disks in the RAID group, andthe appropriate caching of parity information with respect to thestriped data. In the example of a WAFL based file system and process, aRAID 4 implementation is advantageously employed. This implementationspecifically entails the striping of data across a group of disks, andseparate parity caching within a selected disk of the RAID group.

Each filer “owns” the disks that comprise the volumes that the filerservices. This ownership means that the filer is responsible forservicing the data contained on the disks. If the disks are connected toa switching network, for example a Fibre Channel switch, all of thefilers connected to the switch are typically able to see, and read from,all of the disks connected to the switching network. However, only thefiler that owns the disks can write to the disks. In effect, there is a“hard” partition between disks that are owned by separate filers thatprevents a non-owner filer from writing to a disk.

This ownership information is stored in two locations. This ownership ofdisks is described in detail in U.S. patent application Ser. No.10/027,457 ENTITLED SYSTEM AND METHOD OF IMPLEMENTING DISK OWNERSHIP INNETWORKED STORAGE, which is hereby incorporated by reference. In theexample of a WAFL based file system, each disk has a predeterminedsector that contains the definitive ownership information. Thisdefinitive ownership sector is called sector S. In an exemplaryembodiment, sector S is sector zero of a disk. The second source of thisownership information is through the use of Small Computer SystemInterface (SCSI) level 3 reservations. These SCSI-3 reservations aredescribed in SCSI Primary Commands-3, by Committee T10 of the NationalCommittee for Information Technology Standards, which is incorporatedfully herein by reference.

The combination of sector S and SCSI-3 reservation ownership informationis often displayed in the following format <SECTORS, SCSI>, whereSECTORS denotes the ownership information stored in sector S and SCSI isthe current holder of the SCSI-3 reservation on that disk. Thus, as anexample, if sector S and the SCSI-3 reservation of a disk both show thatthe disk is owned by a filer, arbitrarily termed “Green”, that disks'ownership information could be denoted <G,G>, where “G” denotes Green.If one of the ownership attributes shows that the disk is un-owned, a Uis used, i.e. <G,U> for a disk whose SCSI-3 reservations do not show anyownership.

The need often arises to transfer the ownership of a volume from onefiler to another filer in a switch-connected network. This need canarise, when, for example, one filer becomes over-burdened because of thenumber of volumes it is currently serving. By being able to transferownership of a volume or a set of disks from one filer to another, filerload balancing can be accomplished. Currently, if a volume is to betransferred from one filer to another, the disks that comprise thevolume need to be physically moved from one filer to another.

Accordingly, it is an object of the present invention to provide asystem and method for transferring ownership of a volume in a networkedstorage arrangement. The system and method should be atomic (i.e., alldisks are transferred, or none of the disks are transferred), andmaintain the consistency of the disks.

SUMMARY OF THE INVENTION

This invention overcomes the disadvantages of the prior art by providinga system and method for transferring volume ownership from one filer toanother filer without the need for physically moving the diskscomprising the volume. A two-part transfer process is for transferringvolume ownership is employed, with various embodiments for logging orfor when a filer is not active.

According to one embodiment, the source filer modifies the two ownershipattributes from a source-owned state to a completely un-owned state. Thedestination filer then modifies the un-owned disks' ownership attributesto a destination-owned state.

In another illustrative embodiment, both the destination and sourcefilers maintain log files that are updated after each step in thetransfer process. If the transfer process is interrupted by, forexample, one of the filers becoming inactive, the logs can be utilizedto continue the process when both filers are active.

In another embodiment, if the filer that currently owns the volume isnot active, the destination filer first transfers the disks from thesource-owned state to an un-owned state. The destination filer thentransfers the disks from the un-owned state to a destination-ownedstate.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and further advantages of the invention may be betterunderstood by referring to the following description in conjunction withthe accompanying drawings in which like reference numerals indicateidentical or functionally similar elements:

FIG. 1 is a schematic block diagram of a network environment includingvarious network devices including exemplary file servers having filersand associated volumes;

FIG. 2 is more-detailed schematic block diagram of an exemplary fileserver in accordance with FIG. 1;

FIG. 3 is a schematic block diagram of a storage operating system foruse with the exemplary file server of FIG. 2 according to an embodimentof this invention;

FIG. 4 is a schematic block diagram of a network environment includingexemplary filers and disks connected to a switching network;

FIG. 5 is a block diagram of the various steps of the transfer processin accordance with an embodiment of this invention;

FIG. 6 is a flow chart detailing the steps of the transfer process whenthe source filer is alive;

FIG. 7 is a flow chart of the transfer process when the source filer isnot alive;

FIG. 8 is a flow chart of the transfer process including logging whenthe source filer is alive;

FIG. 9 is a flow chart of the transfer process including logging whenthe source filer is not alive; and

FIG. 10 is a flow chart of the steps of the Repair( ) process.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

A. Network Environment

FIG. 1 is a schematic block diagram of an exemplary network environment100 in which the principles of the present invention are implemented.The network 100 is based around a local area network (LAN) 102interconnection. However, a wide area network (WAN), virtual privatenetwork (VPN) (utilizing communication links over the Internet, forexample), or a combination of LAN, WAN and VPN implementations can beestablished. For the purposes of this description the term LAN should betaken broadly to include any acceptable networking architecture. The LANinterconnects various clients based upon personal computers 104, servers106 and a network cache 108. Also interconnected to the LAN may be aswitch/router 110 that provides a gateway to the well known Internet 112thereby enabling various network devices to transmit and receiveInternet based information, including e-mail, web content, and thealike.

Exemplary file servers 114, 116 and 118 are connected to the LAN 102.These file servers, described further below, are configured to controlstorage of, and access to, data in a set of interconnected storagevolumes. As described further below, each file server is typicallyorganized to include one or more RAID groups of physical storage disksfor increased data storage integrity and reliability. Each of thedevices attached to the LAN include an appropriate conventional networkinterface arrangement (not shown) for communicating over the LAN usingdesired communication protocols such as the well known Transport ControlProtocol/Internet Protocol (TCP/IP), User Datagram Protocol (UDP),Hypertext Transfer Protocol (HTTP), or Simple Network ManagementProtocol (SNMP).

The file servers are also connected to a switching network 122 utilizingan appropriate form of switching technology. For example, the switchingnetwork can be a Fibre Channel link. It is expressly contemplated thatother forms of switching networks can be utilized in accordance withthis invention. A plurality of physical disks 124, 126, 128, 130 and132, which comprise the volumes served by the filers are also attachedto the switching network 122. Thus, any file server can access any diskconnected to the switching network.

B. File Servers

FIG. 2 is a more-detailed schematic block diagram of the exemplary fileserver 114, implemented as a network storage appliance, such as aNetApp® filer available from Network Appliance, that is advantageouslyused with the present invention. Other filers can have similarconstruction (including filers 116 and 118). By way of background, afile server, embodied by a filer, is a computer that provides fileservice relating to the organization of information on storage devices,such as disks. However, it will be understood by those skilled in theart by the inventive concepts described herein may apply to any type offiler, whether implemented as a special-purpose or general-purposecomputer, including a stand alone computer. The filer comprises aprocessor 202, a memory 204, a network adapter 206 and a storage adapter208 interconnected by a system bus 210. The filer also includes astorage operating system 212 that implements a file system to logicallyorganize the information as a hierarchical structure of directories andfiles on the disks.

In the illustrative embodiment, the memory 204 may have storagelocations that are addressable by the processor and adapters for storingsoftware program code or data structures associated with the presentinvention. The processor and adapters may, in turn, comprise processingelements and/or logic circuitry configured to execute the software codeand manipulate the data structures. The storage operating system 212,portions of which are typically resident in memory and executed by theprocessing elements, functionally organize a file server by inter-aliainvoking storage operations in support of a file service implemented bythe file server. It will be apparent to those skilled in the art thatother processing and memory implementations, including variouscomputer-readable media, may be used for storing and executing programinstructions pertaining to the inventive technique described herein.

The network adapter 206 comprises the mechanical, electrical andsignaling circuitry needed to connect the file server to a client overthe computer network, which as described generally above can comprise apoint-to-point connection or a shared medium such as a local areanetwork. A client can be a general purpose computer configured toexecute applications including file system protocols, such as the CommonInternet File System (CIFS) protocol. Moreover, the client can interactwith the file server in accordance with the client/server model ofinformation delivery. The storage adapter cooperates with the storageoperating system 212 executing in the file server to access informationrequested by the client. The information may be stored in a number ofstorage volumes (Volume 0 and Volume 1), each constructed from an arrayof physical disks that are organized as RAID groups (RAID GROUPs 1, 2and 3). The RAID groups include independent physical disks includingthose storing a striped data and those storing separate parity data(RAID 4). In accordance with a preferred embodiment RAID 4 is used.However, other configurations (e.g., RAID 5) are also contemplated.

The storage adapter 208 includes input/output interface circuitry thatcouples to the disks over an I/O interconnect arrangement such as aconventional high-speed/high-performance fibre channel serial linktopology. The information is retrieved by the storage adapter, and ifnecessary, processed by the processor (or the adapter itself) prior tobeing forwarded over the system bus to the network adapter, where theinformation is formatted into a packet and returned to the client.

To facilitate access to the disks, the storage operating systemimplements a file system that logically organizes the information as ahierarchical structure of directories in files on the disks. Eachon-disk file may be implemented as a set of disk blocks configured tostore information such as text, whereas the directory may be implementedas a specially formatted file in which other files and directories arestored. In the illustrative embodiment described herein, the storageoperating system associated with each volume is preferably the NetApp®Data ONTAP operating system available from Network Appliance, Inc. ofSunnyvale, Calif. that implements a Write Anywhere File Layout (WAFL)file system. The preferred storage operating system for the exemplaryfile server is now described briefly. However, it is expresslycontemplated that the principles of this invention can be implementedusing a variety of alternate storage operating system architectures.

C. Storage Operating System

As shown in FIG. 3, the storage operating system 212 comprises a seriesof software layers including a media access layer 302 of network drivers(e.g., an Ethernet driver). The storage operating system furtherincludes network protocol layers such as the IP layer 304 and its TCPlayer 306 and a UDP layer 308. A file system protocol layer providesmulti-protocol data access and, to that end, includes support from theCIFS protocol 310, the Network File System (NFS) protocol 312 and theHTTP protocol 314.

In addition, the storage operating system 212 includes a disk storagelayer 316 that implements a disk storage protocol such as a RAIDprotocol, and a disk driver layer 318 that implements a disk accessprotocol such as e.g., a Small Computer System Interface (SCSI)protocol. Included within the disk storage layer 316 is a disk ownershiplayer 320, which manages the ownership of the disks to their relatedvolumes. A disk migration level 322 is a subset of the disk ownershiplevel 320. Notably, the disk migration level 322 works to change on-diskreservations and sector S ownership information.

Bridging the disk software layers with the network and file systemprotocol layers is a file system layer 324 of the storage operatingsystem. Generally, the file system layer 324 implements the WAFL filesystem having an on-disk file format representation that is a blockbased. The WAFL file system generated operations to load/retrieve therequested data of volumes if it not resident “in core,” i.e., in thefile server's memory. If the information is not in memory, the filesystem layer indexes into the inode file using the inode number toaccess an appropriate entry and retrieve a logical block number. Thefile system layer then passes the logical volume block number to thedisk storage/RAID layer, which maps out logical number to a disk blocknumber and sends the later to an appropriate driver of a disk driverlayer. The disk driver accesses the disk block number from volumes andloads the requested data into memory for processing by the file server.Upon completion of the request, the file server and storage operatingsystem return a reply, e.g., a conventional acknowledgement packetdefined by the CIFS specification, to the client over the network. Itshould be noted that the software “path” through the storage operatingsystem layers described above needed to perform data storage access forthe client received the file server may ultimately be implemented inhardware, software or a combination of hardware and software (firmware,for example).

FIG. 4 is a schematic block diagram showing an illustrative switchingnetwork 400 with file servers and disks attached thereto. In thisexemplary configuration there is a “green” file server 402 which ownsgreen (G) disks 404, 406, and 408. Red file server 410 owns red (R) disk412. The blue file server 414 owns the blue (B) disks 416 and 418. Eachset of disks is allocated to a particular region. The regions aredefined as the set of disks that a particular file server owns. Forexample, the green file server 402 has exclusive write access to thegreen disks 404, 406, and 408. Red file server 410 may read data fromthe green disks 404, 406, and 408, but may not write therein.

This write protection is generated by the use of data written on eachdisk's sector S and through SCSI-3 reservations. In the illustrativeembodiment, the data written on sector S is the definitive ownershipdata for a particular disk. The SCSI-3 reservations are generated viathe SCSI protocol as previously described.

D. Volume Transfer

FIG. 5 shows the steps of the transfer process in accordance with thisinvention. In this example, Disks 1, 2 and 3 are currently owned by theGreen file server and are to be transferred to the Red file server. Theinitial state shows disks 1, 2 and 3 owned by the green file server.Both the sector S data and the SCSI-3 reservations are labeled as green(i.e. <G,G>). Step one of the transfer process (TP1) is to convert thedisks from the initial state into a completely un-owned (U) stage(<U,U>). There are two variants of step one. In step 1 a, the sector Sinformation is modified to an un-owned state (<U,G>) and then the SCSI-3reservations are changed to the un-owned state, resulting in <U,U>. Step1 b involves first changing the SCSI-3 reservations to an un-owned state(<G,U>). The second part of step 1 b is changing the sector S data to anun-owned state. At the end of step 1 a or 1 b the disks will becompletely un-owned, i.e. <U,U> and at the intermediate step.

Step 2 of a transfer process (TP2) involves modifying the disks from theintermediate state <U,U> to a state signifying their ownership by thered file server <R,R>. There are also two alternate methods ofperforming step 2 of the transfer process. Step 2 a involves firstwriting the SCSI reservation data to the disks (<U,R>) and then writingthe sector S data. Step 2 b involves first writing the sector S data(<R,U>) and then writing the SCSI reservation data to the disks. At theend of either step 2 a or 2 b, the result will be a disk completelyowned by the red file server (<R,R>). When the disks are in a <R,R>state the transfer process has completed the transfer of ownership.

In addition to marking the sector S area as un-owned, the transferprocess may also annotate the disk. This annotation can, in oneembodiment, be stored in sector S. The annotation can include the volumename that the disk belongs to and the names of the source anddestination filers that are participating in the movement of the volume.An annotation permits an un-owned disk to be distinguished from otherun-owned disks in a networked storage system.

FIG. 6 is a flow chart showing the steps of the transfer processperformed by both the destination file server and the source file serverif the source file server is alive. In step 605, the destination fileserver sends Message 1 (M1) to the source file server. M1 contains aninitial request for transferring a specific volume. In response to M1,the source file server calls the function Verify_Source( ) (step 610) tosee if the source file server can release the volume about that has beenrequested to be migrated. For example, if the volume requested is a rootvolume for the file server it cannot be migrated.

The source file server then sends, in step 615, Message 1Acknowledgement (M1 ack), which contains the list of disks in the volumeand other required information if the volume can be transferred. If thevolume cannot be transferred M1 ack will be an abort message. Thedestination file server checks M1 ack at step 620 to see if it containsan abort message. If an abort message is found, the destination fileserver aborts the transfer at step 625. If M1 ack is not an abortmessage, the destination file server calls the Verify_Dest( ) function(step 630) to verify that a destination file server can acquire thevolume requested. For example, the destination file server must have allrequired licenses associated with the volume to be transferred.

If the destination file server can accept the volume to be transferred,it sends Message 2 (M2) to the source file server at step 635. If thedestination file server cannot accept the volume to be migrated, M2contains an abort message. The source file server verifies the contentsof M2 (step 640). If it is an abort message the file server aborts thetransfer as in step 645. Otherwise, the source filer calls theVerify_Source( ) function (step 647) to determine if the volume is stilleligible to be released. The source file server will off-line the volumeto be transferred at step 650. In accordance with step 655, the sourcefile server will conduct step one from the transfer process. As thesource file server is alive, it is preferable that step 1 a be utilizedas the first step of the transfer process (TP1). After the completion ofTP1, the source file server sends an acknowledgement Message 3 (M3) tothe destination file server (step 660). The destination file server upona receipt of M3 performs step 2 from the transfer process (TP2) at step665. After the completion of TP2, the transfer is complete (step 670)and the destination file server may attempt to bring the volume on-line.

FIG. 7 is a flow chart of the steps performed by the destination fileserver if there is no response to M1. If there is no response to M1, itis assumed that the source file server is dead. In step 705, thedestination file server calls the Verify_Source( ) function to verifythat the volume can be transferred. The destination file server thencalls the Verify_Dest( ) function (step 710) to ensure that it hasappropriate licenses, permissions or other required aspects. The fileserver then performs step one of the transfer process (TP1) in step 715.As the source file server is not alive the destination file server usesstep 1 b of the transfer process by first changing the reservations andthen writing new ownership information to sector S. Upon the completionof TP1, at step 720, the destination file server then performs step 2 ofthe transfer process (TP2). The volume has completed its transfer uponthe completion of TP2 (725).

FIG. 8 is a flow chart detailing the steps that the source anddestination file servers will perform when the logging transfer processis used and the source file server is alive. The destination file servercommences the transfer sending message M1 (step 806) to the source fileserver. Message M1 contains the name of the volume to be transferred.Upon receiving M1 the source file server performs Verify_Source( ) (step809) to determine whether the volume can be transferred. If the volumecan be transferred, the source file server, at step 815, sends M1 ackwhich contains a positive acknowledgement. If Verify Source( ) fails, anabort message is encapsulated in M1 ack. The destination file serverthen verifies the contents of M1 ack in step 818. If M1 ack contains anabort indicator, the transfer is aborted in accordance with step 821.Otherwise, upon receipt of M1 ack, the destination file server moves tostep 824 and performs VerifyDest( ) to determine whether the volumeshould be transferred. If the check succeeds, the destination fileserver records destination log record one (LD1) at step 827 and thenmoves to step 830 where it sends M2 to the source file server asking thesource file server to initiate the transfer. If Verify_Dest( ) failsthen M2 contains an abort indicator.

Upon receipt of a positive confirmation in M2, the source file serverre-executes the Verify_Source( ) function (step 837). If the disks arestill eligible to be transferred, the source file server then writes afirst source log record (LS1) noting the start of the transfer at step839. The source file server then performs step one of the transferprocess (TP1) at step 842. As the source file server is alive, it ispreferable that step 1 a be used as TP1. Upon completion of TP1, thesource file server commits a second source log record (LS2) (step 845)and then sends message M3 at step 848 to the destination file servernotifying the destination file server of the completion of the sourceside transfer. When the destination file server receives M3 it commits asecond destination log record (LD2) at step 851 noting the destinationfile server's receipt and then sends the source file server anacknowledgement through M3 ack (step 854). The destination file servermoves to step 860 and performs step two of the transfer process (TP2).After completion of TP2, at step 863, the destination file server erasesits log entry for this transaction, thereby completing (step 866) thetransfer. When the source file server receives M3 ack, it erases its logentry for the transaction at step 857. It should be noted that steps 857and 860-866 can occur concurrently.

This logging protocol does not assume a reliable transfer process. Thisimplies messages can be dropped due to a network or recipient problem,and therefore messages must be retransmitted under time outs in certainstates. The log records at each node denote its state in the transferprocess. State changes occur, and new log records are written, whenmessages are received. The table below tabulates the states of the nodesagainst possible events and lists the appropriate response in each case.

TABLE 1 Event State Timeout M1 M1ack M2 M3 M3ack LD1 Retransmit X X XMove to X M2 LD2 LD2 Ignore X X X Retransmit X M3ack LS1 Ignore X XIgnore X X LS2 Retransmit X X Retransmit X Move to Default M3 M3 DefaultIgnore Source_Chk Destina- Move to LS1 Retransmit Ignore Send M1acktion_Chk. state. M3ack with Send M2 if error successful.

If the file server determines, while initializing, that it is in theprocess of a part of the transfer process, it then completes theremaining steps. There are several special rules applying to thecontinuation of the transfer process. If the destination file serverreboots and detects a log entry LD3, the destination file server repeatsstep 2 from the transfer process and then erases the log entry. If asource filer reboots and detects a log entry LS2, it repeats transferprocess step 1 and then commits log entry LS3 and then continues on withthe process as previously described. As was stated before, these stepsshould be performed before the final list of owned disks is arrived atand passed on to RAID 4 for assimilation. Subsequent to booting, thefile server should check the presence of any log entries correspondingto this protocol and assume the states implied by them and start anytimers as required. A recovery from a failure during the process of thetransfer process will automatically occur by following the rules in thestate table above.

FIG. 9 is a flow chart detailing the steps that the destination fileserver performs by using a logging transfer process when the source fileserver is not alive. The destination file server first calls theVerify_Source( ) function (step 905) and then Verify_Dest( ) (step 910)to ensure that the volume can be transferred. Then at step 915, thedestination file servers commits log record LR0 marking the beginning ofthe logged transfer process from a dead file server along with a list ofdisks being transferred. At step 920 the file server executes TP1. Asthe source file server is dead, it will execute step 1 b. Uponcompletion of step one, the transfer process the destination file servermoves to step 925 and commits log record LR1. The file server thenexecutes TP2 at step 930. Upon completion of the TP2, the file servercompletely erases the log record at step 935 at which point (step 940)the transfer is complete.

If the destination file server fails at a given point in the middle ofthis operation, it will determine from the presence of log records whatthe status of the ongoing operation was the next time it boots. Thereare three possible scenarios from the destination file server reboots:

1. No log record is found;

2. Log record LR0 is found;

3. Log record LR1 is found.

If no log record is found, nothing needs to be done as no disk ownershipinformation has been modified. If log record LR0 is found, then the fileserver needs to execute step one of the transfer process commit logrecord LR1 and then execute step two of the protocol. If log record LR1is found, then the destination file server needs to execute step twofrom the protocol and the erase the log record upon completion. Asdescribed above, these recovery actions should be performed as part ofthe boot process preferably before RAID initializes to infer accuratelythe set of disks that the file server is suppose to own.

The use of the logging protocols ensures atomicity in those cases wherea filer crashes during the transfer but then later recovers. However, ifthe crashed filer never recovers, atomicity is not guaranteed. To ensureatomicity in those cases, a separate procedure should be called. Thisprocedure, arbitrarily called Repair( ), takes a filer's identificationas a parameter and returns a list of volumes that are completely orpartially owned by that filer. FIG. 10 is a flow chart detailing thesteps of an exemplary Repair( ) function. It should be noted that othermethods of performing this function are expressly contemplated.

The Repair( ) function first selects all unclaimed disks that have areservation set to the specified filer (step 1010). For example, ifFiler A was the desired filer, all disks of the form <X,A> would beselected, where X can be any designation. Next, in step 1020, thefunction selects all unclaimed disks that have no reservation, but havean annotation identifying that disk as one being moved to or from thespecified filer. Then, the function selects all disks whose sector Sinformation identifies the disk's owner as the specified file server(step 1030). Finally, in step 1040, the Repair( ) function determinesall volumes that are partially or completely owned by the specified fileserver.

This determination made in step 1040 is the result of the functionrunning a RAID assimilation algorithm over the pool of disks. The RAIDassimilation algorithm organizes the disks into a set of logical unitsthat can be utilized by both the file system and RAID. In anillustrative WAFL-based file system, the assimilation process organizesthe disks into RAID groups, volumes and mirrors (if mirroring isactive). The assimilation routine will generate a listing of volumesthat are partially owned by a specified file server and partiallyunowned. This information can then be output to the administrator foruse in determining whether volumes should be moved from a dead filer.

As an example, assume that Filer A is dead. The system administratorexecutes Repair( ) with Filer A as a parameter. If the function returnsthe information that volume X is partially owned by Filer A, theadministrator may want to move (using TP_D or L_TP_D) volume X to filerthat is alive.

The foregoing has been a detailed description of the invention. Variousmodifications and additions can be made without departing from thespirit and scope of this invention. Furthermore, it expresslycontemplated that the processes shown and described according to thisinvention can be implemented as software, consisting of acomputer-readable medium including program instructions executing on acomputer, as hardware or firmware using state machines and the alike, oras a combination of hardware, software and firmware. Accordingly, thisdescription is meant to be taken only by way of example and not tootherwise limit the scope of this invention.

1. A method of transferring ownership of a volume having a plurality ofdisks from a source server to a destination server, the methodcomprising the steps of: changing a first attribute of ownership fromsource server ownership to an un-owned state by writing the change to alog data structure and rewriting the first attribute of ownership on thedisk, where the first attribute is a predetermined ownership sector oneach disk; changing a second attribute of ownership from sourceownership to an un-owned state by writing the change to a second logdata structure and rewriting the second attribute of ownership on thedisk, where the second attribute is small computer systems interface(SCSI) reservation; changing the first attribute of ownership from theun-owned state of ownership to destination server ownership by writingthe change to a third log data structure and rewriting the firstattribute of ownership on the disk; and changing the second attribute ofownership from the un-owned state to destination server ownership bywriting the change to a fourth log data structure and rewriting thesecond attribute of ownership on the disk.
 2. The method of claim 1,further comprising: in the event of a failure during the process oftransferring ownership, utilizing the log data structures to continuethe process of changing ownership.
 3. A system to transfer ownership ofa volume having a plurality of disks from a source server to adestination server, comprising: means for changing a first attribute ofownership from source server ownership to an un-owned state by writingthe change to a log data structure and rewriting the first attribute ofownership on the disk, where the first attribute is a predeterminedownership sector on each disk; means for changing a second attribute ofownership from source ownership to an un-owned state by writing thechange to a second log data structure and rewriting the second attributeof ownership on the disk, where the second attribute is a small computersystems interface (SCSI) reservation; means for changing the firstattribute of ownership from the un-owned state of ownership todestination server ownership by writing the change to a third log datastructure and rewriting the first attribute of ownership on the disk;and means for changing the second attribute of ownership from theun-owned state to destination server ownership by writing the change toa fourth log data structure and rewriting the second attribute ofownership on the disk.
 4. The system of claim 3, further comprising: inthe event of a failure during the process of transferring ownership,means for utilizing the log data structures to continue the process ofchanging ownership.
 5. A system to transfer ownership of a volume havinga plurality of disks from a source server to a destination server,comprising: a first computer to change a first attribute of ownershipfrom source server ownership to an un-owned state by writing the changeto a log data structure and rewriting the first attribute of ownershipon the disk, where the first attribute is a predetermined ownershipsector on each disk; a second computer to change a second attribute ofownership from source ownership to an un-owned state by writing thechange to a second log data structure and rewriting the second attributeof ownership on the disk, where the second attribute is a small computersystems interface (SCSI) reservation; a third computer to change thefirst attribute of ownership from the un-owned state of ownership todestination server ownership by writing the change to a third log datastructure and rewriting the first attribute of ownership on the disk;and a fourth computer to change the second attribute of ownership fromthe un-owned state to destination server ownership by writing the changeto a fourth log data structure and rewriting the second attribute ofownership on the disk.
 6. The system of claim 5, further comprising: inthe event of a failure during the process of transferring ownership, acomputer to utilize the log data structures to continue the process ofchanging ownership.
 7. The system of claim 5, further comprising: thefirst computer, the second computer, the third computer, and the fourthcomputer are a single computer.
 8. The system of claim 7, furthercomprising: the single computer is the destination server.
 9. The systemof claim 5, further comprising: the first computer and the secondcomputer are the source server.
 10. The system of claim 5, furthercomprising: the third computer and the fourth computer are thedestination server.